saprophyte -- Organism which feeds on dead and decaying organisms, allowing the nutrients to be recycled into the ecosystem. Fungi and bacteria are two groups with many important saprophytes.

saxitoxin -- neurotoxin found in a variety of dinoflagellates. If ingested, it may cause respiratory failure and cardiac arrest.

scanning electron microscope (SEM) -- n. A special kind of microscope that scans samples with a high-energy beam of electrons to produce a high-resolution, detailed, three-dimensional image. An SEM can magnify a sample up to 250 times that of the best light microscopes.

scavenger -- An organism that feeds upon dead and dying organisms.

sea-floor spreading -- n. The process of adding to the Earth's crust at mid-ocean ridges as magma wells up and forces previously formed crust apart.

secondary growth -- Growth in a plant which does not occur at the tips of the stems or roots. Secondary growth produces wood and bark in seed plants.

sedentary -- Living in a fixed location, as with most plants, tunicates, sponges, etc. Contrast with motile.

sediment -- Any solid material that has settled out of a state of suspension in liquid.

sedimentary rock -- Any rock resulting from the consolidation of sediment.

seed -- A structure produced by seed plants which encapsulates the embryo. The seed often provides nourishment during germination, but may lie dormant for many years first.

segmentation -- In many animals, the body is divided into repeated subunits called segments, such as those in centipedes, insects, and annelids. Segmentation is the state of having or developing a body plan in this way.

selection -- Process which favors one feature of organisms in a population over another feature found in the population. This occurs through differential reproductionthose with the favored feature produce more offspring than those with the other feature, such that they become a greater percentage of the population in the next generation.

septum -- Partition which divides up a larger region into smaller ones, such as in the central body cavity of some anthozoa.

sexual reproduction -- A type of reproduction in which two parents give rise to offspring that have unique combinations of genes inherited through the gametes of the two parents. Sexual reproduction involves meiosis and syngamy.

shoot -- Usually, the above ground portion of a plant, bearing the leaves. Contrast with root.

Siberia -- n. A separate continental plate that existed from the Latest Precambrian to the Carboniferous, composed of a large part of central Russia, namely Siberia.

siderite -- Also called ironstone, that is a concretion of iron carbonate. Common in the Mazon Creek fossil beds.

silica -- amorphous silicon dioxide (glass). It is a structural component in many organisms, such as diatoms and horsetails.

silicification -- Process whereby silica replaces the original material of a substance. For example, silicified wood.

sill -- n. A sheet-like igneous intrusion that parallels the plane of the surrounding rock.

sinkhole -- n. A natural depression in the surface of the land caused by the collapse of the roof of a cavern or subterranean passage, generally occurring in limestone regions.

siphon -- Opening in molluscs or in urochordates which draws water into the body cavity. In many molluscs, the siphon may be used to expel water forcibly, providing a means of propulsion.

siphonostele -- When a plant's vascular tissue develops as a central cylinder, it is said to have a siphonostele. See also protostele and eustele.

sister group -- The two clades resulting from the splitting of a single lineage.

skeleton -- Support structure in animals, against which the force of muscles acts. Vertebrates have a skeleton of bone or cartilage; arthropods have one made of chitin; while many other invertebrates use a hydrostatic skeleton, which is merely an incompressible fluid-filled region of their body.

sporangium -- A chamber inside of which spores are produced through meiosis.

spore -- n. A single cell that is dispersed as a reproductive unit, or that encapsulates a cell during unfavorable environmental conditions; in organisms with an alternation of generations; the products of meiosis are spores.

sporophyll -- Any leaf which bears sporangia is called a sporophyll.

sporophyte -- The diploid stage in the life cycle of an organism undergoing an alternation of generations. The sporophyte is multicellular and develops from a zygote. The mature sporophyte meiotically produces haploid spores that later generate the gametophyte generation.

stamen -- Part of a flower, the tip of which produces pollen and is called the anther. More info?

starch -- a complex polymer of glucose, used by plants and green algae to store surplus sugar for later use.

stasis -- A period of little or no discernible change in a lineage.

stem group -- All the taxa in a clade preceding a major cladogenesis event. They are often difficult to recognize because they may not possess synapomorpies found in the crown group.

stigma -- The sticky tip of a pistil. Or, the dense region of pigments found in many photosynthetic
protists which is sensitive to light, and thus functions somewhat like a miniature eye.More info?

stipe -- A scientific term for "stalk".

stipules -- Paired appendages found at the base of the leaves of many flowering plants.

stomata -- Openings in the epidermis of a stem or leaf of a plant which permit gas exchange with the air. In general, all plants except liverworts have stomata in their sporophyte stage.

strike -- The direction or trend of a bedding plane or fault, as it intersects the horizontal.

strobilus -- A tightly clustered group of sporophylls arranged on a central stalk; commonly termed a "cone" or "flower".

style -- The narrow stalk of the pistil, located above the ovary but below the stigma.

subduction -- n. A geologic process in which one edge of one crustal plate is forced below the edge of another; subduct– v.; subduction zone- n. A long narrow area in which subduction is taking place, e.g. the Peru-Chile trench, where the Pacific Plate is being subducted under the South American Plate.

subsidence -- n. The sudden sinking or gradual downward settling of the Earth’s surface with little or no horizontal motion.

substrate -- "Supporting surface" on which an organism grows. The substrate may simply provide structural support, or may provide water and nutrients. A substrate may be inorganic, such as rock or soil, or it may be organic, such as wood.

sugar -- any of several small carbohydrates, such as glucose, which are "sweet" to the taste.

symbiosis -- n. A relationship between two organisms that live in intimate contact with each other; includes mutualism (both organisms benefit, they rely on each other for survival), parasitism (one organism benefits at its host's expense) and commensalism (one partner benefits and the other is neither benefitted nor harmed); symbiotic- adj.

synangium -- A cluster of sporangia which have become fused in development.

synapomorphy -- A character which is derived, and because it is shared by the taxa under consideration, is used to infer common ancestry.

synapsid -- n. A vertebrate distinguished by a skull with one pair of openings in the sidebehind the eyes, e.g., mammals and their close relatives.

syncline -- A fold of rock layers that is convex downwards. Antonym of anticline.